Good day

I'm reading through this article about the ID3v2 tag, so that I can easily read the tags of an MP3 file once I've understood it all. But I'm kind of stuck on one point and I was hoping someone could shed some light over this for me.

I understand that the header of the ID3v2 tag looke like this:

Code:
ID3v2/file identifier   "ID3" 
ID3v2 version           $03 00
ID3v2 flags             %abc00000
ID3v2 size              4 * %0xxxxxxx
Im able to extract all parts of this header except for the size tag. This is what the article tells be about the size tag:

The ID3v2 tag size is encoded with four bytes where the most significant bit (bit 7) is set to zero in every byte, making a total of 28 bits. The zeroed bits are ignored, so a 257 bytes long tag is represented as $00 00 02 01.

Im not sure how to "convert" these 4 bytes to an integer taking what is written above into consideration. I've tried to simply construct an integer from these 4 bytes, but the resulting value is just too large. Some advice would be great